1. Field of the Invention
The present invention concerns a method of manufacturing a cylindrical glass part such as a rod or a tube, for example a method of manufacturing a cylindrical optical fiber preform.
2. Description of the Prior Art
Some glasses, including fluorinated glasses, are difficult to process and in particular ally physical contact with this kind of glass during shaping introduces impurities and results in the formation of crystallites or microcrystals; all such defects compromise the optical quality of the glass.
Given these difficulties, conventional glassmaking techniques are not able to produce in a single operation a cylindrical part which has the following properties simultaneously:
excellent optical quality, because the glass constituting it is homogeneous, PA0 perfect surface state, PA0 extremely well controlled inside and outside diameters (accurate to within 50 .mu.m or better), PA0 lengths exceeding 100 mm. PA0 a vertical cylindrical cavity is lined with a porous material and its inside dimensions are a few tens of microns greater than those of said part, PA0 a cylindrical pedestal constituting a support for said part is adapted to slide in said cavity, PA0 an initial seed mass constituted from said glass is provided on said pedestal, PA0 said seed mass is heated until it melts and a gas is injected permanently into said porous material in order to maintain between them and said molten seed mass a layer of gas a few tens of microns thick to prevent any contact, PA0 said molten seed mass is fed continuously from the top of said cavity with the powder constituents of said glass, and PA0 said pedestal is lowered as said cylindrical part is formed from said seed mass. PA0 the pedestal is a hollow cylindrical body closed at the bottom by a piston, PA0 the pedestal initially contains a solid glass charge adapted to form the seed mass, and PA0 the molten seed mass is extracted progressively from the pedestal by pushing the piston upwards. PA0 the cavity is tubular and lined with coaxial porous membranes, and PA0 the pedestal is annular. PA0 the cavity is tubular and hollow, and PA0 the pedestal is tubular.
French patent FR-A-2 649 392 discloses a technique for producing a fluorinated glass rod using gas film confinement.
This technique has not previously been applied to the manufacture of a tube because of specific difficulties due to the much more complex shape.
An object of the present invention is to adapt this technique to the manufacture of a tube, for example a fluorinated glass tube.
Another object of the present invention is to improve this technique in the case of manufacturing a rod.